In humans it is approximately 2.5 to 4 cm long, extending from the anorectal junction to the anus. It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the lumen closed in the form of an anteroposterior slit.

It is differentiated from the rectum by the transition of the internal surface from endodermal to skinlike ectodermal tissue.